Preheat oven to 350F. Grease and flour a 9×5″ loaf pan.
In a small mixing bowl, whisk together flour, baking powder and salt.
In a medium mixing bowl, beat all four egg whites to soft peaks. Beat in 2 tablespoons of the brown sugar, one tablespoon at a time, until sugar is fully incorporated. Egg whites should still be at soft or somewhat stiff peak stage. If in doubt, err on the side of caution and don’t overbeat. Set aside.
In a large mixing bowl, cream together butter and remaining brown sugar (1 cup minus 2 tbsp). When fluffy, beat in all of the egg yolks and the vanilla extract. With the mixer on low speed, gradually beat in the flour mixture. When all the flour has been incorporated, beat in 1/3 of the egg whites. When smooth and with the mixer still on low, beat in another 1/3. Fold in remaining egg whites with a large spatula, making sure no streaks of egg whites remain visible in the batter. Pour into prepared pan.
Bake for 40-60 minutes, until a tester comes out clean.
Cool in pan for 5-10 minutes, then turn out onto a wire rack to cool completely before slicing